1. Field of the Invention
The present invention concerns a method to create magnetic resonance (MR) images (in particular in the field of MR angiography and for the presentation of perfusion information) and a magnetic resonance system designed to implement this method, as well as a corresponding non-transitory computer data storage medium.
2. Description of the Prior Art
In MR angiography operating without contrast agent it is known to operate with spin labeling (a marking of the spin). For this purpose, a slice-shaped volume (i.e. a volume bounded in a single spatial dimension) that contains the vessels of interest is normally marked with the use of a slice-selective inversion band. From a geometric standpoint, the slice-shaped volume or the slice has two flat, uncurved planes that are parallel to one another as boundary surfaces. A slice is excited with a conventional frequency-selective RF pulse with a constant magnetic field gradient. This means that a lateral boundary does not exist in addition to the boundary surfaces (boundary planes) of the slice. However, in practice the examination subject (the patient) or an imaging volume of the magnetic resonance system is finite, such that the slice-shaped volume also has a lateral boundary somewhere.
Measurement signals are acquired after the inversion time (TI “Inversion Time”) and are translated into corresponding MR angiography images. The spins within the volume ideally yield no signal (or at least only a weak a signal) while fluids (blood, for example) flowing into the vessels from outside the volume generate a relatively high signal strength; see for example “Free-breathing renal MR angiography with steady-state free-precession (SSFP) and slab-selective inversion: initial results”; M. Katoh et al.; Kidney Int. 2004; 66(3); Pages 1272-1278 and “Selective visualization of renal artery using SSFP with Time-Spatial Labeling Inversion Pulse Non-Contrast Enhanced MRA for patients with renal failure”; Y. Yamashita et al.; Proc. Intl. Soc. Mag. Reson. Med. 13 (2005), Page 1715.
However, for patients with a low cardiac output or for regions with a slow blood flow, it is difficult to sufficiently fill the vascular tree of interest with fresh, unsaturated incoming blood, in particular given a short TI. Those parts of the vascular tree that are not reached by the fresh, unsaturated, incoming blood disadvantageously remain dark in the resulting MR angiography images. That blood that flows into the vascular tree from the inverted, slice-shaped volume generates no or nearly no signal due to the preceding inversion of its spins, and therefore shortens the length of the visible portion of the vascular tree within the MR angiography images.
The fact that inverted or saturated blood generates nearly no signal can also be used advantageously. For example, in the abdominal region a volume is typically inverted that travels well beyond the imaging volume in the caudal direction (in the direction of the feet). Venous blood that flows into the imaging volume thus is also inverted and (as is most often desired) suppressed. However, the described effect is unwanted with regard to the arterial blood that flows into the vascular tree to be examined. In order to minimize the problem, according to the prior art (for example) the boundary surface of the inversion slice is placed optimally close to (flush with) the vascular tree to be examined in the direction from which the blood flows.
Nevertheless, the problem is sometimes disruptive, in particular in an examination of the renal arteries with MR angiography images, wherein the slice-shaped inversion volume has to include both kidneys. Since both kidneys are situated in the slice-shaped inversion volumes, a situation inevitably arises that a significant quantity of the arterial blood “disappears” in the aorta, since this blood is located within the inversion volume (as is shown in illustrations (a), (b) and (c) in FIG. 2).
In illustration (a) in FIG. 2 a point in time is shown shortly after the spins within the inversion volume 26′ have been inverted. It is apparent in illustration (a) in FIG. 2a that the slice thickness 32 of the inversion volume 26′ is greater than the slice thickness of the imaging volume 30 (which also applies for the other illustrations in FIG. 2, as well as the illustrations in FIGS. 3 and 4).
The situation of an inversion time (TI) of 750 ms after the inversion is shown in illustration (b) in FIG. 2. Within this 750 ms, in a healthy patient (with normal cardiac output) a large amount of blood has already flowed from outside the inversion volume 26′ into the section of the aorta 29 that is located within the inversion volume 26′, and therefore also into the vascular tree 23, such that in the case of the acquisition of an MR angiography image the corresponding portion of the vascular tree 23 is visible (in black) in illustration (b) in FIG. 2.
If the MR angiography image is only acquired 1500 ms after the inversion (as is shown in illustration (c) in FIG. 2), for a healthy patient nearly the entire vascular tree 23 is visible. It also occurs that a subset of the blood that has flowed in has then flowed out of the imaging volume 30 again, as is the case in the lower section of the aorta 29 in illustration (c) in FIG. 2.
The venous blood supply 33 is also shown in illustrations (a), (b) and (c) in FIG. 2. By selecting the distance between the lower (in FIG. 2) boundary surface of the inversion volume 26′ and the lower edge of the vascular tree 23 (the lower boundary surface of the imaging volume 30) is to be relatively large, nearly no venous, unsaturated blood flows into the imaging volume 30 even given an inversion time of 1500 ms.
With reference to illustration (a), (b) and (c) in FIG. 2, it is noted that for many medical questions the visibility of the vascular tree 23 beginning at the ostium 25 (i.e. the point at which the vascular tree 23 branches away from the aorta) up to the peripheral branchings plays a significantly more important role than the visibility of the aorta 29 itself.
While the vascular tree 23 is clearly visible at least at an inversion time of 1500 ms for a patient with normal cardiac output, according to the prior art this can not be the case for a patient with low cardiac output (as is shown as an example in illustrations (a), (b) and (c) in FIG. 3). Due to the lower cardiac output, in illustration (b) in FIG. 3 (TI=750 ms), the unsaturated blood has still not even flowed up to the ostium 25. Although the unsaturated blood has also flowed into the forward section of the vascular tree 23 in illustration (c) in FIG. 3 (TI=1500 ms), it has not yet flowed into the peripheral branches, causing these to be nearly invisible in the MR angiography image represented by illustration (c) in FIG. 3.
In order to ensure the visibility of (optimally) the entire vascular tree (up to the peripheral branchings), even in patients with a low cardiac capacity, according to the prior art an optimally large inversion time (TI) is used in addition to the arrangement described above of the boundary surface of the inversion slice being optimally close to the vascular tree to be examined. However, this procedure has disadvantages. For long inversion times with a correspondingly low cardiac output, it frequently occurs that at least the peripheral branchings are not visible in the MR angiography images. Moreover, an extension of the inversion time (TI) inevitably leads to a long pulse sequence repetition (TR; “Time to Repetition”) in order to ensure a sufficient elimination of the background signals. However, even given a very long repetition time (TR), the background signals can no longer be optimally suppressed given a long inversion time, such that the quality of the generated MR angiography images suffers.